Attach or associate a file to a Customer in Deluge?

Attach or associate a file to a Customer in Deluge?

I am trying to connect a Customer in Books with a Contact in CRM, specifically an attachment that is found in the CRM record.

In CRM, I attach the file in Deluge with the zoho.crm.attachFile call, however, I do not have that option in Books, it seems. I noticed in Books, there is a central "Files" area where attachments are stored and it seems they are referenced to whatever object in Books they are meant to be associated with, though I could not find a way to upload there from Deluge either. I see emailing the attachment as an option, but I see no automation possible to associate the file with a Customer.

I'm out of ideas, and I would love to hear yours if you have any, thanks!